In many industrial processes, such as paper making, it is of importance to know and to be able to control the relatively weak voltages which for various and as yet partly not completely understood reasons are generated in flowing particle suspensions. It has been found--see for instance the paper by W. Sack: "Kontinuierliche Stromungspotentialmessung an einer Papiermaschiene" in Das Papier 30, Nr 10A, V42-V46 (1976)--that the potentials in question may be measured by diverting from the process a sample, having the sample flow through a screen which traps most of the particles forming on the upstream side of the screen a pad of particles, and measuring, by means of two electrodes positioned on each side of the screen, the voltage differential between the upstream and the downstream side of the screen, viz. the voltage generated in the pad of particles when the liquid flows through the pad. The invention relates to a method and an appartus for carrying out measurements of this kind or for carrying out measurements of the streaming potential and/or the state of drainage of a particle suspension.
In the above paper by Sack there are also described a method and a device for measuring the streaming potential of a particle suspension. However, the apparatus is of an experimental nature intended, in the first place, for the study of the technique in question and is not suited for the highly continuous measurements required at reasonable costs for the control of the streaming potential desired in an industrial process. EP No. 0 079 726 A1 discloses an apparatus for carrying out measurements partly on the basis of and according to the contents of the above paper. Neither does this apparatus satisfactorily provide for continuous measurement as it is not, to a desirable degree, exempt from the effects of electrical disturbances, and it is unnecessarily complicated because it requires a special pump and measurements of the pressure caused by this pump. Also, it has the disadvantage that it does not permit the satisfactory disposal of waste in connection with the necessary rinsing operations.
It is a known fact that the streaming potential is proportional to the dielectric constant of the liquid phase, the pressure differential between the upstream and downstream sides of the above-mentioned particle pad, the specific conductivity of the liquid phase, and the viscosity of the liquid phase. It is also a known fact that the streaming potential and the drainage property affect each other, and hence it would be of advantage to be able to measure both simultaneously to allow the streaming potential to be corrected for the state of drainage and vice versa.